The
ignition system is an integral part of any engine's performance, just a much as
the fuel delivery is, whether it is carbureted or fuel injection.

We will start out with the basic operation
of the early HEI (High Energy Ignition) as it debut in mid 1974 and work toward
the ESC (electronic spark control) as it appeared in 1981. But first the basics.

Courtesy of GM

The standard parts for a HEI distributor that replaced
the breaker plate and points in the previous distributor:

Timer core

Pole piece

Ignition control module

Permanent magnet and pick up coil

The HEI system controls the primary current in the
ignition to time the spark via a magnetic signal of the pick up coil. When the
teeth are aligned, the magnetic field increases, which induces a electric
current through the pick up coil windings. AS the timer core and pick up coil
move away from each other, the magnetic field collapses and the voltage signal
is diminished. The magnetic field and pick up coil generate an AC
(alternating current) in the pick up coil leads and the windings.

Courtesy of GM

When the pick up coil voltage falls below a certain
threshold, the ignition module opens the primary current flow within the module.
The ground path is eliminated from the coil primary windings, collapsing the
magnetic field. The voltage that is induced in the coil (secondary) circuit is
the result of the mutual induced current from the magnetic field collapsing.

The diagram above is typical of the early HEI using 4
pin ignition module and vacuum advance, non EST 1974-1980.

BASIC HEI OPERATION- Electronic Spark
Control

The ECM uses other electronic devices to help
control the spark timing under various conditions (such as under load, deceleration,
etc). This includes Electronic Spark Timing and later with the conjunction with
Electronic Spark Control (ESC) to control detonation. The basic principle
to control the primary (coil) current by a transistorized ignition module is
also used with Distributorless Ignition System (DIS).

There are two modes to the EST timing. Bypass, which is
basically base timing ( no computer controlled), when the engine is being
cranked, or running below 400 rpm. The second mode, is EST, which the computer
has control over the timing (during normal operation). Base timing is
not fixed, there is some advance engineered into the module circuit.

In bypass mode (shown above), the pick up coil is the
trigger signal for the ignition coil. When the engine is cranking below a
predetermined threshold (400-600 rpm), the computer has no control over the
timing. Only when it reaches the threshold speed, does the computer take over
timing.

The set timing connector is used to take the timing
control away from the computer. This is used only when setting the
"base" or mechanical timing of the engine. Depending on the
year/model. this may mean shorting the "A" and "B" terminals
on the ALDL (diagnostic terminal).

In EST mode (shown above), when the engine reaches the
threshold speed, the ecm applies a 5 volt signal to the ignition bypass
circuit. This signal switches the bypass "off" and turns on the
EST circuit for triggering the ignition coil. Spark timing is now controlled by
the ECM based on information programmed in the "chip" and input
signals from the MAP, MAF, engine coolant temperature and engine rpm
speed.

Some engine use a hall effect switch, which used
used in place of the reference terminal on the HEI module. The Hall effect
switch is added to the primary ignition circuit to permit use of the Electronic
Spark Timing. On other engines and applications, the Hall effect switch is used
to trigger the fuel injection and used in camshaft timing reference for the
position of the number one cylinder (4.3 CPI and CSFI engines, 5.0 and 5.7L Gen
I Vortec V8 engines).

The Hall effect switch is a electronic circuit which
emits a voltage signal controlled by the presence or absence of a magnetic
field.

The switch is mounted in the distributor above the
normal pick up coil/module assembly. It contains a small magnet that has a
specified gap between it and the electronic switch. One or four steel
vanes (depending on application) are attached to the distributor shaft. As the
shaft rotates, it alternately blocks and unblocks the electronic circuit and the
output signal is used as the reference signal to the ECM.

Ignition voltage is supplied on ignition module
terminal "B" at 12VDC+. This voltage is passed on through the
transistor (in direction of arrow) through a resistor. The collector is
connected to ground. The base of the transistor is connected to the transducer,
which is the conversion of magnetic energy to electrical. When a magnetic force
is induced (not blocked by the shield) the transistor is in its "on"
state and voltage passes through the transistor. This will put the distributor
reference at less than 1 volt. The resistor in the power supply line at the
ignition module terminal "B" will keep the voltage biased at this turn
on voltage. When the magnetic force is blocked with the vane between the
magnet and the transducer, there is no voltage induced into the switching
transistor, thus it will not conduct any voltage through the transistor and
therefore is switched "off". The reference voltage is not being
"grounded" close to 0 volts, and therefore it is at near battery
voltage.

With the ignition key on, engine not running, a voltage
will be present at the output of the switch, whether less than 1 volt or close
to 12 volts and that will depend on the position of the vane in the
distributor.

When the engine is running, the output voltage measured
will be approximately 6 volts. With the reference line to the ECM from the Hall
effect is open, the ECM has no distributor pulses and also it will not deliver
no injector pulses.

Electromagnetic Interference

Electromagnetic Interference or EMI for short, is a phenomenon
that affects sensitive electronic equipment. This includes HEI, EST and the ECM.
When high current carrying conductors such a spark plugs are routed too close to
the ignition wiring for the EST controls, coil trigger harness. This will induce
false signals on the reference line to the ECM. This in turn commands the ECM to
send false and erroneous information to the EST system. This will adversely
affect the timing control and injector pulses. This can the engine to miss or
stall in dangerous situations.

So it is important to keep as much distance from the
spark plug wires to the EST wiring, plug in connectors and wiring harness. It is
important the clips and straps are used to keep the harness wiring from being
loose and potentially coming in close contact with high current carrying
conductors. The same can apply to the O2 sensor wiring as well, as it will cause
problems in fuel delivery (too rich or too lean, stumbling and stalling).